Study of the Mechanical Interactions between the Gas Diffusion Layer and the Flow Field Structure in Polymer Electrolyte Fuel Cells (PEFCs)

Kurzfassung

In Polymer Electrolyte Fuel Cells (PEFCs) the reaction- and product-gases and liquids are transported in the channels of the flow field structure as well as in the pores of the gas diffusion layers (GDLs). In contrast to stiff backing materials (e.g. carbon paper) softer ones like carbon cloth are compressed over the ribs of the gas distributor or impressed into the channels when the PEFC is assembled. During fuel cell optimisation the interactions between the different fuel cell components, specially the important one between the gas diffusion layer and the flow field, are frequently neglected; hence flow fields as well as gas diffusion layers are commonly optimized independently. The DLR has investigated these interactions with a two-stage approach: At first, the basic influence of the mechanical compression was analysed by measuring the generated current and the local current density distribution in the potentiostatic mode as a function of the contact pressure. These investigations were carried out with a segmented laboratory cell with an active area of 25 cm2. The overall performance as well as the local current density distribution shows a strong dependency on the compression state of the cell. Thereby, an optimal contact pressure exists, at which the influences of the contact resistance and the mass transfer are minimized. In a second step, the impression depth of the gas diffusion layer into the flow field channels was determined as a function of the contact pressure by three different methods: mechanically, optically and electronically. For the measurement of the depth of impression optically and electronically an appropriate device was developed that consists of a copper plate with spark-eroded channels, ridges of acrylic glass therein and the gas diffusion layer on the ribs. While the outer appearance of this device is equal to the gas distributor of the fuel cell, this set-up acts as a parallel connection of different capacities. Accordingly, the depth of impression in the gas diffusion layer can be determined by measuring the change of the electrical capacity if the material is pressed into the channel structures.